Ion-control method for electrochemical machining

ABSTRACT

Method of and apparatus for electrochemically machining a workpiece wherein the machining electrolyzing current passes in the form of steep-wavefront pulses on one polarity spaced by intervals and during these intervals, opposite-polarity pulses are applied across the tool electrode and the workpiece with a pulse width at most equal to the duration of the respective interval but preferably of a shorter duration and with an adjustable lag.

United States Patent [72] Inventor Kiyoshi lnoue 100 Sakato, Kawasaki,Kanagawa, Japan [21] Appl. No. 714,251

[22] Filed Mar. 19, 1968 [45] Patented Oct. 26, 1971 [32] PrioritiesMar. 20, 1967, Mar. 28, 1967, Mar. 30,

1967, Mar. 31, 1967, Apr. 18, 1967 [33] Japan [22 Filed Mar. 19, 1968[45] Patented Oct. 26, 1971 Continuation-impart of application Ser. No.475,375, July 28, 1965.

[54] ION-CONTROL METHOD FOR ELECTROCHEMICAL MACHINING 12 Claims, 21Drawing Figs.

[52] U.S. Cl 204/143 M, 204/228 [51] Int. Cl B23p 1/00 {50] Field ofSearch 204/143 M,

References Cited UNITED STATES PATENTS 3,475,312 10/1969 lnoue 204/2173,420,759 1/1969 lnoue 204/143 3,407,125 10/1968 Fehlner 204/2283,294,666 12/1966 Wiersma 204/141 3,223,603 12/1965 lnoue 204/228FOREIGN PATENTS 1,093,114 11/1967 Great Britain 204/143 PrimaryExaminer-John H. Mack Assistant Examiner-Sidney S. Kanter Allorney- KarlF. Ross ABSTRACT: Method of and apparatus for electrochemicallymachining a workpiece wherein the machining electrolyzing current passesin the form of steep-wavefront pulses on one polarity spaced byintervals and during these intervals. opposite-polarity pulses areapplied across the tool electrode and the workpiece with a pulse widthat most equal to the duration of the respective interval but preferablyof a shorter duration and with an adjustable lag.

PATENTEDUET 25 I971 3,516,34

sum 3 CF 5 FIG. 6 W W all/l4 BY YL g. (LRDSS ATTORNEY ION-CONTROL METHODF OR ELECTROCHEMICAL MACHINING This application is acontinuation-in-part of application Ser. No. 475,375,filed July 28,1965.

My present invention relates to an ion-control system forelectrochemically machining a conductive workpiece and represents afurther development of the technique originally described in my U.S.Pat. No. 3,357.9l2.

In the patent, l have described an apparatus for machining a conductiveworkpiece as well as a method making use of such apparatus wherein thecurrent applied to the machining gap is periodically reversed fordepolarization and depassivation of the surfaces of the system. Asobserved in that patent, one 'of the problems arising in electrochemicalmachining systems is that ion contamination occurs along the surface ofthe tool juxtaposed with the workpiece and/or the formation of an oxidefilm along the workpiece. As a consequence, a process termed"passivation occurs in the electrode gap which must be countered byvarious means. In my copending application, Ser. No. 475,375, filed 28July 1965 for example, I have described one method of eliminating suchpassivation whereby spark discharge breaks up a passivating film in acavity-sinking arrangement, the passivation film being purposelygenerated to protect portions of the workpiece at which no machining isto occur. In this case, the depassivation or activation of thejuxtaposed surfaces of the tool and workpiece makes use of high-energymechanical electrical shock waves to destroy the film. Others havepointed out that passivation may be avoided or eliminated by usinghigh-pressure highvelocity streams of electrolyte in a relatively narrowgap. Systems of this latter type have been found inconvenient from thepoint of view of the hardware necessary to carry out machining undersuch conditions and the sensitivity of the system to changes in theelectrolyte pressure. Vibration has also been proposed as a possiblesolution to this problem. Furthermore, certain materials are not readilymachinable because of their chemical composition by a continuous currentsystem. For example, tungsten carbide requires polarity reversalperiodically for optimum machining (see U.S. Pat. No. 3,357,912). Insubstantially all systems which have provided periodic reversal of theelectrolytic machining current have applied out-of-phase AC or pulsatingDC signals in superimposition upon the machining current so that themachining current waveform and the reversal waveform both may beconsidered generally sinusoidal or at best rounded with nonsteep leadingand trailing flanks. Only the amplitude of the machining signals and thereverse signals can be adjusted in these devices.

lt is the principal object of this invention to provide a system for theelectrochemical machining of a conductive workpiece, especiallycomposite (e.g. tungsten-carbide workpieces which are difficult tomachine) which gives rise to an improved machining rate, bettermachining accuracy and decreased passivation at the gap.

Another object of the present invention is to provide a system of thecharacter described which can be used without difficulty for differentmaterials whose optimum machining parameters may differ.

These objects and others which will become apparent hereinafter, areattained in accordance with my present invention which provides a powersupply for delivering a series of machining pulses to the electrode andworkpiece with an intervening interruption of the signal, a reversalpulse being applied during such reversal and having essentially steep orsquare wave flanks which are adjustable with respect to the timing andcorrelation with the trailing edge of a prior machining pulse and theleading flank of the following machining pulse. It has surprisingly beenfound that when the reversal pulse has its leading flank substantiallycoincident with the trailing flank of the machining pulse, the machiningprocess is most effective with iron bodies; however, when tungstenbodies are machined, best results are obtained when the trailing edge ofthe reversal pulse coincides with the leading edge of the subsequentmachining pulse. The machining of copper and copper-zinc alloys by theelectrochemical method of the present invention is best carried out witha reversal pulse substantially midway between the machining pulses.

Accordingly, I provide a power supply with solid-state switching devicesand control both of the machining-pulse duration and interpulse intervalduration and reversing switching means for generating a narrow pulse(pulse width less than the interval width) with adjustable timing sothat the initiation of the reversal pulse can be simultaneous withtermination of a machining pulse or may be delayed with respect totermination of a preceding machining pulse or may terminate concurrentlywith initiation of a subsequent machining pulse in dependence upon thematerial to be machined. l have found that a power supply for thispurpose best comprises a number of parallel-connected solid-stateswitching devices (eg transistors) in series with a DC source and themachining system, two such circuits being provided with DC sourcesoppositely poled for generating the machining pulses and the reversalpulses respectively. According to a more specific feature of thisinvention, a multivibrator timing circuit is provided for alternatelyactivating and deactivating the respective sets of switching transistorsvia amplifying transistors or the like, preferably tied to a timingconstant network establishing the pulse duration. Between the output ofthe multivibrator and the parallel-connecting switching transistorsassigned to the negative pulse train, I provide a controllabletime-delay network whose time constant can be reduced to zero but whichotherwise establishes a lag between cutoff of the machining pulse andinitiation of the reversal.

As noted earlier, a further but related discovery in connection withdepassivation of the electrochemical machining system is thatsteep-flank substantially square wave signals at the machining regionprovide a sharp increase in the accuracy of the machining operation asdetermined by reproducibility of the electrode or tool shape in theworkpiece. As a practical matter, even though a square wave pulse isapplied across the electrode and the workpiece, it is found thatpassivation films develop during the single machining pulse at arelatively high rate causing a decrease in the current during thepassage of each machining pulse. The resulting current versus time plotof the waveform shows a sloping rounded shape as the passivation filmdevelops, this shape being repeated in the machining pulses of the trainas a consequence of reformation of the passivation film after eachreversal pulse. l have found that the effects of such passivation filmsduring the passage of the machining pulse can be reduced sharply byshaping the machining pulse to constitute it as a variable signalchanging substantially at the rate of formation of the passivation filmand adapted to maintain the current across the machining gapsubstantially constant.

When reference is made herein to electrochemical machining and thepresence of a machining gap, it must be understood that theseexpressions include electrochemical grinding wherein, as described in mycopending applications. Ser. No. 5l2,338 (now U.S. Pat. No. 3,475,3 l2)and 562,857 (now U.S. Pat. No. 3,429,759), filed 8 Dec. I965 and 5 Julyi966, every effort is made to urge the electrode tool against thesurface of the body to be machined. Various principles of the powersupply system described above have also been applied in my copendingapplications, Ser. No. 511,827 (now U.S. Pat. No. 3,527,686) and682,824, filed 6 Dec. I965 and 14 Nov. 1967, respectively. Thewaveform-shaping network for the machining-voltage pulses and thereversal pulses (if necessary) may be inductive or capacitiveimpedances, L-C differentiating or integrating networks or simply R-Cpulse shapers.

The above and other objects, features and advantages of the presentinvention will become more readily apparent from the followingdescription, reference being made to the accompanying drawing in which:

FIG. 1 is a circuit diagram of an apparatus embodying the presentinvention;

FIGS. 2A-2C are waveform diagrams showing the preferred current/timerelationship for the machining and reversal pulses of the presentinvention;

FIG. 3 is a graph representing the results obtained with a specificexample of the invention;

FIGS. 4A and 4B represent the relationship between voltage and currentand the effects of passivation films;

FIGS. SA-SE show the improved waveforms of the present invention;

FIG. 6 is a circuit diagram of another arrangement in accordance withthis invention;

FIG. 7 is a diagram of a further system using improved feedback;

FIGS. 8, 9 and 10 represent other circuits for energizing theelectrochemical machining system of the present invention;

FIGS. 11 and 12 are waveform diagrams resulting from the circuits ofFIGS. 8-20;

FIG. 13 is a circuit diagram of another power supply embodying thepresent invention; and

FIG. 14 is a wave diagram of the machining pulses produced with thissystem.

In FIG. 1, I show a circuit for operating an electrochemical grindingapparatus of the general type described in my copending application,Ser. No. 512,335, filed 8 Dec. 1965, and having a contoured wheel 10composed of graphite or the like and driven by a motor 10a. Theelectrolyte is supplied to the interface between the electrode 10 andthe metallic workpiece 12 by a nozzle 11 supplied with the electrolyteby a pump 11a from a filter 11b and a collecting vessel 11c. It will beunderstood, however, that the present invention applies equally toelectrochemical cavities sinking and tap removal, to electrochemicalmachining using rodlike or elongated electrodes, etc.

In accordance with the present invention, the machining pulse isdelivered across the machining gap by a first series circuit constitutedby an adjustable DC source 13, and a bank of switching transistors 15.In this system, the switching transistors 15a, 15b and 15c have theiremitter-collector branches connected in parallel with one anotherbetween the battery 13, one terminal of which is connected to the toolelectrode 10, and the workpiece 12 so that, when transistors 15a and 15care rendered conductive, they apply the positive pulse (FIGS. 2A2C)serving for the principal machining operation. The bases of transistor15a-l5c are connected via the usual biasing resistor to the emitterterminal of a NPN transistor 19T whose function will be described ingreater detail hereinafter. An adjustable bias resistor 15d establishesthe base-collector bias while resistors l5e establish the emittercollector bias for the transistors Isa-15c.

The negative pulse is generated by a switching network 16 in series withan adjustable DC source 14 between the electrode 10 and the workpiece12, the sources 13 and 14 being poled oppositely to one another. Here,the transistors l6a-l6c have the emitter-collector terminals in paralleland bridged by a bias resistor 16d and are energized via biasingresistors at their base terminals from the output transformer 16L of amultivibrator power supply and timer. In accordance with this invention,a multivibrator network 18 is provided with a pair of transistors I8Tand 1ST cross-coupled via adjustable resistors 18R,, 18R and adjustablecapacitors 18C and 18C in conventional flip-flop configuration, theswitching device being energized by the battery 18B. Output or loadresistors 18R, and 18R, are also provided.

The output signal of the multivibrator developed across 18R has aduration T(+)=kR,C which represents the duration of the positive pulsewhere k is a constant and R and C represent the resistance andcapacitance of the resistor 18R and 18C,, respectively. This signalenergizes the output transistor 19T to apply a further signal across thevoltagedividing resistor 19R and thereby trigger the transistors 150 forthe duration of the machining pulse, the RC network 18R,, 18C thereafterswitching the multivibrator 18 to block transistor 19T.

The signal developed across resistor 1811,, whose duration isrepresented by the relationship T(-)=kR C (where R; and C are theresistance and capacitance of members 18R and 186,, respectively)triggers the output transistor 19T'. This transistor energizes aunijunction transistor timer 190 via a delay network 10d which may becut out entirely by the switch 19d. The delay network comprises anadjustable resistor 19d"in parallel with a capacitor 1911". After anadjustable delay period determined by the time constant of this network,the unijunction oscillator 190 is energized to provide an output at thetransformer 16L to trigger the switching circuit 16 for a perioddetermined by the constancy of the relaxation network 190', 19o"whichare, respectively, a variable resistor connected between the emitter andone base of the unijunction transformer 1914 and a capacitor connectedbetween the emitter and the other base of the unijunction transformer19a and a capacitor connected between the emitter and the other base ofthe unijunction transistor.

In FIGS. 2A-2C, I have illustrated several waveforms which have beenfound satisfactory for the machining of iron-tungsten carbide and copperor copper-zinc alloys, respectively. In each of these Figures, theamplitude of the current (fl) is plotted along the ordinate against timeas the abscissa. The duration of the positive pulse is represented atT(+) while the interval between the positive pulses is indicated at T().At a delay period D or D (FIGS. 28 and 2C) which may equal zero (FIG.2A) determined by the network 19d, the reversal pulse is generated bythe switching network 16 for a period R where. in accordance with anessential feature of this invention R T() and R+D T(). Preferably D isabout 20 msecs. The current ofFIGS, 2A,2B. 2C,5B,5C and 5D is in theform of steep-wavefront substantially instantaneously triggered andcutoff constant-level pulses of one polarity spaced by intervals andhaving during said intervals substantially instantaneously triggered andcutoff constant-level opposite-polarity pulses having a pulse with lessthan the duration of the respective interval.

EXAMPLE I Using the apparatus in FIG. 1, a tungsten carbide workpiececontaining 6 percent by weight cobalt was electrochemically ground in anaqueous potassium nitrate (5 percent by weight) electrolyte over amachining area of 1.6 cm using the waveform represented in FIG. 2B and adelay period D of 20 msec. The results obtained are plotted in FIG. 3.The electrode was composed of graphite. In FIG. 3, the duration R of thenegative pulse in msec. is plotted along the abscissa while threeseparate ordinate plots represent the ratio of electrode to workpiecewear in percents, the roughness of the machining surface in #(I'lmax)and the machining rate in g./min. The machining rate is represented as adot-dash line while the electrode wear is shown by broken lines and thesurface roughness in solid lines. When R=0 (corresponding to no reversalof current and merely a 20 msec. interruption), the ratio of electrodewear to workpiece wear (E/W) is about 10 percent while the surfaceroughness is about 6 p.(Hmax) and the machining rate is approximately0.8 g./min. with a negative spike of a duration of 3 to 5 msec., themachining rate is raised to substantially l.2 g./min. while theelectrode wear is reduced to its minimum of about 2 percent (E/W) whilethe surface roughness is reduced to about 0.2-1 p.(I-Imax). Thereafter.the electrode wear increases, the machining rate falls while the surfaceroughness remains substantially constant. Surprisingly, as R approachesT(-), analogous to the waveform used in my patent No. 3,357,9l2, theelectrode wear rises sharply, the surface roughness remains constant orincreases slightly depending upon the materials used and the machiningrate falls off sharply as well. Furthermore, the sharper the wave frontsof the signal, the greater is the reproducibility of themachining'process and the reproduction of the machining surface.Waveforms of the type shown in FIGS. 2A and 2C are most suitable for usewith iron and steel workpieces and with copper and copper-zinc alloys,respectively. As noted earlier, I have found that even during thepositive pulse, the passivation film may impede machining or distortsame. Thus, in FIGS. 4A and 48, I have plotted the voltage applied by asquare wave generator with intervening reversal along the ordinateagainst time along the abscissa while the current is likewiserepresented in broken lines. The voltage is shown in solid lines. I havealready pointed out that, preferably, the machining current waveformshould be a square wave. However, the square wave is precluded byformation of the passivation film which, although the voltage maintainsits square wave form, assumes a sawtoothlike configuration withmachining current loss as represented by hatching in FIGS. 4A and 4B. Inboth cases, the broken lines represent the actual machining currentwhile the dot-dash line represents the preferred current level for themachining operation. I have found that the effects of the passivationfilm, which appears to reform at each machining pulse (FIG. 4A) or formssubstantially automatically and then is destroyed during the machiningpulse (FIG. 413), can be obviated by shaping the machining pulse so asto impart to the current waveform a compensation designed to regeneratethe substantially square wave fonn mentioned earlier. Typical shapedwaves, according to the present invention, are represented in FIGS.5A-5E. In FIG. 5A, for example, I show a waveform which compensates forthe passivation effect illustrated in FIG. 4A. In this system, a pulseshaping is effected to provide a gradual increase in the voltage withtime substantially at the rate necessary to compensate for the currentdecrease with time shown in FIG. 4A. The resulting current waveform(dotted line 1,, in FIG. 5A) thus has the square wave configurationindicated to be desirable. The application of these principles to thewaveforms shown in FIGS. 2A and 2B are subjected to passivation effectsas represented in FIG. 4A so that here, too, I prefer to provide pulseshaping as described in connection with FIG. 5A. The results of suchpulse-shaping are shown in FIGS. 58 and 5C. The application of theprinciple to the system of FIG. 2C is represented in FIG. 5D. The samepulse-shaping principle may be used to decrease the voltage of FIG. 5 13so that the passivation film is destroyed rapidly and a square-type waveconfiguration is imparted to the current fiow when the problem of FIG.4B is encountered.

In FIG. 6, I show a device generally similar to that previouslydescribed but allowing waveform shaping as indicated. In the system ofFIG. 6, the tool electrode 110 is a cavity-sinking member cooperatingwith the workpiece 112 and supplied with electrolyte through theelectrode via the means described in my US. Pat. No. 3,357,912. Amachining pulse is applied across the workpiece/electrode gap from theDC source 113 connected in series with the emitter-collector branches ofparallel-connected transistors 115a, 115b and 115a of solidstateswitching assembly 115. The negative pulses are provided by a DC source114 in series with the emitter-collector terminals of transistors 116a,116b, 1160 of another switching assembly 116. At the output side of themultivibrator trigger 118, which is constructed and operates asdescribed in connection with FIG. 1, there is provided the PNP outputtransistor 119T whose collector lies in series with the voltage dividingresistor 119R. To form the sawtooth voltage waveform represented at FIG.5A, I provide a waveform-shaping impedance (e.g. variable capacitor 119Cwhich is charges ble at a rate determined by the time constant of thenetwork 119C, 119R) to provide a pulse shape as shown at S and triggerthe switching transistors 115A, 1158, SC accordingly. As a consequence,a substantially square wave machining pulse is applied across themachining assembly 110, 112. In place of a capacitive impedance ANINDUC- TIVE impedance may be employed to provide the required pulseshape (FIG. 55).

The other output of the multivibrator 118 is delivered to the base ofthe output transistor 119T which is of the PNP type and is provided inits collector circuit with a delay network 119d whose function has beendescribed earlier. The resistor 1190' and the capacitor 1190" controlthe on"-time of a unijunction transistor 11914 which istransformer-coupled with the switching transistor assembly 116 aspreviously described. The output winding 116L' of the transformer 116Lforms an inductance which, together with a pair of oppositely poledrectifiers 116r' and 1l6r" and a capacitor [16c form an integratingcircuit of spikelike output as represented at S. The spike has asufficient pulse height so that the passivation film is renderedineffective and a square wave pulse is generated during the negativeportion of the cycle as well (FIGS. SA-SD).

According to a further feature of this invention, the reversing pulse isdelivered and the machining pulse is terminated when the machining powerduring each pulse falls to a predetermined level indicative of a filmbuildup to the point that power losses become substantial. Accordingly,I provide a feedback for the timing network which adjusts the intervalbetween machining pulses, whether or not the reversing pulse width iscoincident therewith, thereby triggering at least an interruption of themachining signal and generally also a reversal when the power deliveredto the system during the machining pulse has in part been dissipated bypredetermined buildup of the passivating film. In this embodiment, themultivibrator timer 218 has a variable resistor 218R, forming part of atimeconstant network for controlling the interval T() between machiningpulses.

As represented in this Figure, the wiper of the potentiometer isshiftable by a servomotor, here represented as a solenoid coil 218:. Thepower-detection system includes a resistor 218a in series with theworkpiece, the machining gap 210g and the electrode 210 across the DCsource 213 and the switching transistor assembly 215. The transistors2150, 215b and 2150 have their emitter-collector networks connected inparallel between the source 213 and resistor 2180. Also across theelectrode 210 and the workpiece 212, there is provided a tap whichserves as a feedback of the voltage to the coil 211/: of a Hall-effectassembly 211.

A yoke 211y applies a magnetic field perpendicularly to the Hall effectcrystal 2110 so that the magnetic field is proportional to the amplitudeof the voltage applied across the machining device. A second tap acrossthe shunt resistor 218a passes an electric current through the crystalin a crystal plane perpendicular to the magnetic field while the outputvoltage is tapped perpendicularly to both the magnetic field and theproportional current and is applied across a temporary storage capacitor2110 to the voltage-dividing resistor 2111'.

The reference voltage is supplied by a battery 2115 and is compared withthe voltage developed at resistor 211/: by the coil 218a connected in abridge circuit with the wipers of these variable resistors. The negativepulse generator comprising the variable source of direct current 214 isconnected in series with the tool 210 and with the parallel-connectedemitter-collector networks of the transformers 2160, 1216b and 2160 ofthe switching circuit 216. In this system, the output transistors 219Tand 219T control the voltage tapped from the voltage dividers 219v, 219Vto regulate the duration of the positive pulse'and the interveningpulse. Since the transistor switch 216 is energized directly (i.e.without a delay network or the unijunction transistor timing circuit),the negative pulse is coextensive with the interval between machiningpulses (see FIG. 6A or 613); however, the regenerative feedback from theHall-effect crystal to the servomotor 213s adjusts the period T() aswell as the pulse width R as represented in FIG. 4A. When the filmbuilds up more rapidly, the power variation (resulting from decreasingcurrent while the applied voltage remains constant) will be detectedrapidly and reversal initiated when the current amplitude falls off tothe predetermined minimum level. The unijunction timing network for thenegative pulse (FIG. I), the capacitive or inductive pulse shapingnetworks (FIG. 6) and the LC integrating circuit 116L etc. (FIG. 6) areall compatible with the feedback system illustrated in FIG. 7 and it maybe employed in conjunction therewith.

FIGS. 8-11 represent other circuit arrangements for con trolling thespacing between the terminal flank of the machining pulse and theforward flank of the reversal pulse, the gap width, etc. In FIG. 8, thesystem comprises a DC source 313 whose positive terminal is connected inseries with a solidstate controlled rectifier 3155,, the electrode 310and workpiece 312, a correspondingly poled solid-state controlledrectifier 3158, and a negative terminal of the battery. A similar seriescircuit adapted to effect current flow in the opposite direction, isformed by the positive terminal of battery 313, the solid-statecontrolled rectifier 3158 the workpiece 312, the electrode 310, thesolid-state controlled rectifier 3158, and the negative tenninal of thebattery.

The gates of the series-connected rectifiers are tied for crossoperation in alternation as represented by the dot-dash line (e.g. via amultivibrator); thus the sets M58, and 3158, and 3158,, 3158, aretriggered alternately. When a gap is provided between a switchover ofthe conduction of the 2 sets, current waveforms such as those shown inFIGS. it and 112 are attainable. In FIG. 9, I show a system wherein thepower supply includes a pulse shaping choke 315c between a storagecapacitor 3l3c and its surge-suppressing choke 31131.. The choke 3150facilitates quenching of the controlled rectifiers 3158,, 3158,, 3158and 3158, after these controlled rectifiers have been triggered by themultivibrator 3118. In this system, the multivibrator alternatelyoperates a pair of transformers 319! and 319! each having two secondarywindings. The secondary windings are connected with the respective gatesof the corresponding set of controlled rectifiers to ensure that bothcontrolled rectifiers of each set will be simultaneously energized. INthe modification of FIG. 10, a pair of inductive-capacitive networks isprovided at 31., 31130 and 313L', 3130, bridged across the DC source3E3. A chargelevel detector 313d and 31311 is provided to detect thelevel of charge at the capacitors 3l3c and 313s and, upon the chargelevel attaining a predetermined value, triggering the controlledre'ctifiers 3158,, 3158, and 3158 sass, to discharge the respectivecurrent surge through the controlled rectifiers and across the machininggap. The capacity, charge and discharge times determine the values ofT(+), T(), R and D.

FIG. 13 shows an electrochemical machining circuit operated onprinciples analogous to those described in connection with FIG. 7wherein, however, the interruption of the machining pulse and thecoincidental application of the reversing parts results fromsuperimposing a negative spike upon a continuous DC, the spike amplitudebeing in excess of the continuous-current amplitude. Thus, the apparatuscomprises a graphite electrochemical grinding wheel 411i] which machinesa workpiece 412 with electrolyte being delivered to the interface at411, the workpiece 412 being shifted upon a table 412' in the directionof arrow 412" in a surface-grinding arrangement. A continuous DC source4113 is connected in series with a surge-suppressing choke 413i. and isbridged by a DC blocking capacitor 4l3c while being connected across theelectrode 410 and the workpiece 412. The source 4R3 delivers a machiningcurrent of an amplitude I, to the machining system (see FIG. 14). Thepulse circuit includes a source 414, poled oppositely the source 413 andbridged by a DC blocking capacitor 414a and a voltage divider 414btapped to energizing the relaxation network 4190' and M90" which, inturn, controls a unijunction transistor 41% whose output transformer416L triggers a controlled rectifier M6. A rectifier and inductancenetwork 416q, quenches the controlled rectifier to extinguish thereversing pulse. The unijunction transformer network controls theduration (T of the negative pulse whose amplitude (I exceeds I,.

EXAMPLE 11 Using the circuit of FIG. 13, a tungsten carbide workpiececontaining 3 percent by weight cobalt is machined with a graphite wheeldriven at a speed of 3,000 r.p.m. and having a diameter of 8 inches. Theelectrolyte is a percent aqueous solution of potassium nitrate suppliedat a rate of about 4.5 liter/min. The mean current delivered to themachining system was 60 aJcm. and the amplitude ll was half the pulseamplitude H The machining rate was measured at various ratios ofmachining current on" time (T to machining current off" time ('1,). Witha ratio TJT, of l, machining was carried out at about 0.8 g./min. at aratio TJl switched between 2,3,4and 5, the machining rate rose from L2to L7 and then reduced to 1.3 and 0.8 g./min., respectively. Optimummachining was carried out with a system in which the machining currenton+ time was three times the off" or reversal pulse time. Best resultswere found with current density between [0 a./cm. and 300 a./cm.

The invention described and illustrated is believed to admit of manymodifications within the ability of persons skilled in the art, all suchmodifications being considered within the spirit and scope of theappended claims.

I claim:

11. In a method of electrochemically machining a workpiece wherein amachining electrolyzing current is passed through a tool electrodejuxtaposed with a workpiece and displaced relatively thereto in thepresence of an electrolyte to erode electrochemically the workpiece withformation during such passage of machining current of a passivationfilm, the improvement which comprises the steps of applying saidmachining current in the fonn of steep wave front substantiallyinstantaneously triggered and cutoff constant-level pulses of onepolarity spaced by intervals, and applying during said intervalssubstantially instantaneously triggered and cutoff constantlevelopposite-polarity pulses across the electrode and the workpiece of apulse width less than the duration of the respective interval toelectrochemically destroy said film.

2. The improvement defined in claim 1, further comprising the step ofdelaying each of the opposite-polarity pulses for a predetermined periodsubsequent to the termination of the preceding steep-wavefront pulse ofsaid one polarity.

3. The improvement defined in claim 1 wherein said pulses are applied asvoltage signals across said electrode and said workpiece, furthercomprising the step of shaping the voltagesignal envelopes to impart tothe current during the passage at least of said steep-wavefront pulsesof said one polarity of square wave configuration, thereby maintainingthe intensity of said current substantially constant throughout each ofsaid pulses of said one polarity in spite of the presence of said film.

4. The improvement defined in claim 1 wherein said pulses are applied bydelivering a continuous electric current of said one polarity to saidtool electrode and said workpiece at a first amplitude and superimposingupon said continuous electric current a plurality of opposite-polaritysteep-flanked pulses of an amplitude exceeding said first amplitude.

5. The improvement defined in claim 4 wherein said opposite-polaritypulses are spaced apart at intervals in a ratio to the pulse widththereof of substantially 3:1 and said oppositepolarity pulses have anamplitude equal at least to twice the amplitude of said continuouselectric current.

6. The improvement defined in claim 1, further comprising the step ofautomatically detecting the buildup of a passivation film between saidtool electrode and said workpiece, and controlling the duration of saidintervals in response to said detection and to at least one electricalparameter of said machining current as applied across said electrode andsaid workpiece to restrict the buildup of said film during each of saidsteep wave front pulses.

7. The improvement defined inclaim 11, further comprising the step ofselectively setting the pulse width of said steep wave front pulses ofsaid one polarity in dependence upon the material constituting saidworkpiece.

8. The improvement defined in claim 1, further comprising the step ofselectively setting the duration of said intervals in dependence uponthe material constituting said workpiece.

9. The improvement defined in claim ll, further comprising the step ofselectively setting the pulse width of said oppositepolarity pulses independence upon the material constituting said workpiece.

of the subsequent steep wave front pulse of said one polarity independence upon the material constituting said workpiece.

12. The improvement defined in claim 1, further comprising the step ofselectively setting the relative amplitudes of said steep wave frontpulses of said one-polarity and said oppositepolarity pulses independence upon the material constituting said workpiece.

i i i i

2. The improvement defined in claim 1, further comprising the step ofdelaying each of the opposite-polarity pulses for a predetermined periodsubsequent to the termination of the preceding steep-wavefront pulse ofsaid one polarity.
 3. The improvement defined in claim 1 wherein saidpulses are applied as voltage signals across said electrode and saidworkpiece, further comprising the step of shaping the voltage-signalenvelopes to impart to the current during the passage at least of saidsteep-wavefront pulses of said one polarity of square waveconfiguration, thereby maintaining the intensity of said currentsubstantially constant throughout each of said pulses of said onepolarity in spite of the presence of said film.
 4. The improvementdefined in claim 1 wherein said pulses are applied by delivering acontinuous electric current of said one polarity to said tool electrodeand said workpiece at a first amplitude and superimposing upon saidcontinuous electric current a plurality of opposite-polaritysteep-flanked pulses of an amplitude exceeding said first amplitude. 5.The improvement defined in claim 4 wherein said opposite-polarity pulsesare spaced apart at intervals in a ratio to the pulse width thereof ofsubstantially 3:1 and said opposite-polarity pulses have an amplitudeequal at least to twice the amplitude of said continuous electriccurrent.
 6. The improvement defined in claim 1, further comprising thestep of automatically detecting the buildup of a passivation filmbetween said tool electrode and said workpiece, and controlling theduration of said intervals in response to said detection and to at leastone electrical parameter of said machining current as applied acrosssaid electrode and said workpiece to restrict the buildup of said filmduring each of said steep wave front pulses.
 7. The improvement definedin claim 1, further comprising the step of selectivEly setting the pulsewidth of said steep wave front pulses of said one polarity in dependenceupon the material constituting said workpiece.
 8. The improvementdefined in claim 1, further comprising the step of selectively settingthe duration of said intervals in dependence upon the materialconstituting said workpiece.
 9. The improvement defined in claim 1,further comprising the step of selectively setting the pulse width ofsaid opposite-polarity pulses in dependence upon the materialconstituting said workpiece.
 10. The improvement defined in claim 1,further comprising the step of selectively setting the delay between thetermination of one of said steep-wavefront pulses of said one polarityand the commencement of the subsequent opposite-polarity pulses independence upon the material constituting said workpiece.
 11. Theimprovement defined in claim 1, further comprising the step ofselectively setting a delay between the termination of one of saidopposite-polarity pulses and the commencement of the subsequent steepwave front pulse of said one polarity in dependence upon the materialconstituting said workpiece.
 12. The improvement defined in claim 1,further comprising the step of selectively setting the relativeamplitudes of said steep-wavefront pulses of said one-polarity and saidopposite-polarity pulses in dependence upon the material constitutingsaid workpiece.